A 2007 file photo shows Iraqis carrying a poster of top Shia cleric Grand Ayatollah Ali al-Sistani, who issued a call to arms on Friday for Shia in Iraq to fight against Sunni militants who have seized large swaths of territory in the north. (Ali Abu Shish/Reuters)

Iraq's Shia clerical leadership Friday called on all Iraqis to defend their country from Sunni militants who have seized large swaths of territory, and a UN official expressed "extreme alarm" at reprisal killings in the offensive, citing reports of hundreds of dead and wounded.

U.S. President Barack Obama said he is weighing options for countering the insurgency, but warned Iraqi leaders that he would not take military action unless they moved to address the country's political divisions.

Fighters from the al-Qaeda-inspired Islamic State of Iraq and Syria (ISIS) made fresh gains, driving government forces at least temporarily from two towns in an ethnically mixed province northeast of Baghdad. The assault threatens to embroil Iraq more deeply in a wider regional conflict feeding off the chaos caused by the civil war in neighbouring Syria.

Members of Iraqi security forces stand guard during an intensive security deployment in Kerbala, southwest of Baghdad, on Friday, as Sunni Islamist militants gain more ground in Iraq. (Mushtaq Muhammed/Reuters)

The fast-moving rebellion, which also draws support from former Saddam Hussein-era figures and other disaffected Sunnis, has emerged as the biggest threat to Iraq's stability since the U.S. withdrawal in 2011. It has pushed the nation closer to a precipice that could partition it into Sunni, Shia and Kurdish zones.

Iraqi Prime Minister Nouri al-Maliki, whose Shia-led government is struggling to form a coherent response to the crisis, travelled to the city of Samarra to meet with military commanders late Friday, according to state TV.

Fight as part of civic duty

Militants earlier in the week overran military bases and several communities including the second-largest city of Mosul and Saddam's hometown of Tikrit. Samarra, the site of a prominent Shia shrine 95 kilometres north of Baghdad, sits between Tikrit and the capital.

A representative for Grand Ayatollah Ali al-Sistani, the most revered Shia spiritual leader in Iraq, told worshippers at Friday prayers that it was their civic duty to confront the threat.

Different translations for ISIS:

The name in the original is the Islamic State of Iraq and "al-Sham" — an Arabic word that refers to Greater Syria or the Levant. So it is alternately referred to as the Islamic State of Iraq and the Levant, or ISIS or the Islamic State of Iraq and Syria, or ISIS; or other versions of "al-Sham."

— The Associated Press

"Citizens who can carry weapons and fight the terrorists in defence of their country, its people and its holy sites should volunteer and join the security forces," said Sheik Abdul-Mahdi al-Karbalaie, whose comments are thought to reflect al-Sistani's thinking.

He warned that Iraq faced "great danger," and that fighting the militants "is everybody's responsibility, and is not limited to one specific sect or group."

In Geneva, UN human rights chief Navi Pillay warned of "murder of all kinds" and other war crimes in Iraq, and said the number killed in recent days may run into the hundreds, while the wounded could approach 1,000.

Pillay said her office has received reports that militants rounded up and killed Iraqi army soldiers as well as 17 civilians in a single street in Mosul.

Her office heard of "summary executions and extrajudicial killings" as ISIS militants overran Iraqi cities and towns this week, the statement said.

Fighters of the Islamic State of Iraq and Syria celebrate in Mosul, Iraq, on Thursday. ISIS fighters have seized Iraq's second biggest city, as well as other towns and cities north of Baghdad. (Reuters)

"I am extremely concerned about the acute vulnerability of civilians caught in the cross-fire, or targeted in direct attacks by armed groups, or trapped in areas under the control of ISIS and their allies," Pillay said. "And I am especially concerned about the risk to vulnerable groups, minorities, women and children."

Obama did not specify what options he was considering, but he ruled out sending American troops back into combat in Iraq.

Iran signals willingness to fight militants

"We're not going to allow ourselves to be dragged back into a situation in which, while we're there we're keeping a lid on things, and after enormous sacrifices by us, after we're not there, people start acting in ways that are not conducive to the long-term stability and prosperity of the country," Obama said on the South Lawn of the White House.

Sunni insurgents from an al-Qaeda splinter group extended their control this week to an area that include's Iraq's biggest oil refinery in Baiji. (Reuters)

Administration officials said Obama is weighing airstrikes using drones or manned aircraft. Other short-term options include an increase in surveillance and intelligence-gathering. The U.S. also is likely to increase aid to Iraq, including funding, training and both lethal and non-lethal equipment.

Al-Maliki and other Iraqi leaders have pleaded with Washington for more than a year for additional help to combat the growing insurgency.

Neighbouring Shia powerhouse Iran signalled its willingness to confront the growing threat from the militant blitz.

Former members of Tehran's powerful Revolutionary Guard have announced their readiness to fight in Iraq against the Islamic State, the official IRNA news agency reported. Iranian state TV quoted President Hassan Rouhani as saying his country will do all it can to battle terrorism next door.

"The Islamic Republic of Iran will apply all its efforts on the international and regional levels to confront terrorism," the report said Rouhani told al-Maliki by phone.

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